1. Field of the Invention
The present invention relates to a magnetic recording medium. More particularly, it relates to a magnetic recording medium having improved physical characteristics of an adhesive strength and a repeat running durability as well as a electromagnetic transfer characteristic which has a surface smoothness.
2. Description of the Prior Art
A magnetic recording medium usually prepared by coating a magnetic composition of a mixture of a magnetic powder and a resin binder on a substrate such as a resin film. It has been studied to obtain a magnetic recording medium having higher characteristics. Various resin binders used for the preparation of a magnetic recording media have been proposed.
Urethane type resins, vinyl chloride-vinyl acetate copolymer type resins, cellulose type resins, epoxy type resins, phenoxy type resins and vinyl chloride-vinylidene chloride copolymer type resins have been mainly used for the binder. One or two component of the resins has been used.
In the two component type binders, it has been difficult to satisfy with both electromagnetic transfer characteristics and physical characteristics of an adhesive strength and a repeat running durability by controlling a ratio of the two kinds of the resins, because of contradictory characteristics. Thus, satisfactory dual characteristics could not be expected.
Nitrocellulose proposed as a binder has been widely used in view of excellent wettability to a magnetic powder and excellent leveling in the form of a magnetic paint. However, nitrocellulose has a firing property and careful consideration for preventing firing and explosion is needed. It has been required to incorporate 30 to 35 wt.% of isopropyl alcohol as a protective component to nitrocellulose under regulation in order to keep safety in the storage and handling of nitrocellulose composition. On the other hand, when nitrocellulose is combined with a urethane compound, hydroxyl groups of nitrocellulose remained without a nitration are crosslinked with isocyanate groups of the urethane compound. Thus, when nitrocellulose is used as a binder of a magnetic composition, excellent affinity of nitrocellulose to a magnetic powder and excellent kinetic strength resulted by the crosslinking are remarkably effective for improvement of the electromagnetic transfer characteristic and the physical characteristics.
When the crosslinking reaction with isocyanate groups of the urethane compound is utilized, hydroxyl groups of isopropyl alcohol impregnated in nitrocellulose react with isocyanate groups of the urethane compound to reduce a crosslinked degree whereby the effect for improving physical characteristics has been reduced. In order to overcome the disadvantage, it has been proposed to substitute isopropyl alcohol as the protective component by a plasticizer such as dibutyl phthalate or dioctyl phthalate. However, the plasticizer causes weakness of the magnetic layer whereby it has been difficult to attain desired physical characteristics. Moreover, it has been known that nitrocellulose improves a wearing resistance and a running durability whereas nitrocellulose causes inferior adhesive strength as the disadvantage.